Such pulse-driven infrared-radiation sources are often used to determine the concentrations of components of a gas sample. This is done with the aid of a cuvette arrangement in which the gas sample to be investigated is disposed. The radiation source is connected to a pulse generator driven via a clock. By means of the pulse generator, the voltage supply of the radiation source is switched on and off in correspondence to the pulse clock rate. The infrared-radiation pulses penetrate the measuring cuvette with more or less radiation energy being absorbed in dependence upon the concentration of the gas components to be detected. The radiation energy which is not absorbed reaches an energy chamber in which the useful signal is subjected to a further evaluation. The modulation principle is applied for the reason that the measurement radiation can be clearly distinguished from disturbing fluctuations of the radiation intensity. In this way, a better signal-to-noise ratio is obtained. In this connection, reference may be made to German published patent application DE-OS 30 43 332.
In the known circuit arrangement, the time-dependent course of the measurement signal does not only follow the absorption fluctuations corresponding to the changes in concentration in the measurement cuvette; instead, the time-dependent course of the signal is determined by the heating and cooling characteristic of the radiation source which, in turn, is dependent on the pulse power as well as on the pulse duty factor of the pulse sequences. For this reason, the signal maximum and the signal minimum must be filtered out of an unsymmetrically running signal form with the aid of a complex signal processing method. Furthermore, the radiation sources driven with the known intensity modulation are slow and their radiation intensity is dependent upon the ambient temperature, since the peak temperature and also the heating and cooling time constants are influenced by the heat transfer to the ambient.